An Overview of Solar Reflectance Remote Sensing
Methods for Earth Science Applications
S. Platnick
Laboratory for Atmospheres
NASA Goddard Space Flight Center, Greenbelt MD USA
SORCE Science Team Meeting
Sonoma CA
5 December 2003
Outline
•
Solar reflectance remote sensing - a brief overview of passive
solar techniques (excluding UV)
– space-borne/aircraft techniques and instruments
– examples w/emphasis on atmosphere (clouds and aerosols)
•
Radiometric calibration
– radiance vs. irradiance
•
Solar spectral irradiance issues
– use/misuse of irradiance data sets
– 3.7 µm spectral band
Solar Reflectance Satellite Measurement Summary (incomplete)
Example Instruments
Measurement
Heritage
Current/Recent
AVHRR,
Landsat TM,
SPOT
(CNES),
CZCS
MODIS, GLI (JAXA, ADEOSII), ATSR (UK, ERS-1,2),
ASTER (Japan), ETM+,
SeaWiFS, MERIS (ESA,
Envisat)
VIIRS (NPP,
NPOESS)
Directional
MISR (imager),
ATSR, ASTER, POLDER
APS (Glory)
Polarization
POLDER
(CNES, ADEOS-I,II)
APS,
PARASOL
(CNES, A-train)
Spectral,
Spatial
(radiometric
imagers)
Key:
Future
Instrument development/management (other than
Satellite platform
S. Platnick, SORCE, 5 Dec 2003
MODTRAN, absorption transmittance only
H2O =
--O3--
Spectral regions
of interest
O2
O2
VIS
NIR
O2
SWIR
MWIR
SWIR
CO2
CH4
N2O
CO2
S. Platnick, SORCE, 5 Dec 2003
MODTRAN, absorption transmittance only
MODIS (Terra, Aqua)
nominal band locations
– general purpose window bands
(land, aerosol, clouds)
– ocean color/phytoplankton/
biogeo. chemistry
– water vapor bands
cloud particle size
fire detection
S. Platnick, SORCE, 5 Dec 2003
MODTRAN, absorption transmittance only
AVHRR
nominal bands locations
(channel 1, 2, 3)
S. Platnick, SORCE, 5 Dec 2003
MODIS Land Surface Albedo, band 2 (0.86 µm)
global animation for 2001, 16 day averages
(derived from operational product MOD43, E. Moody, et al.)
Click Here to See Movie
QuickTime™ and a
Video decompressor
are needed to see this picture.
S. Platnick, SORCE, 5 Dec 2003
MODIS 0.86 µm
albedo, mid-late
July 2001
QuickTime™ and a
Video decompressor
are needed to see this picture.
MODIS land
classification
map (MOD12)
urban
S. Platnick, SORCE, 5 Dec 2003
MODIS Aerosol Product - daily examples from 2001
(MOD04, L3 1° gridded, Kaufman, Tanre, Remer, et al.)
QuickTime™ and a
Sorenson Video decompressor
are needed to see this picture.
Fine Aerosol
Fraction
Click to See Movie
1.0
0.0
0.25
Aerosol Optical Thickness
0.0
0.5
S. Platnick, SORCE, 5 Dec 2003
MODIS Cloud Product – thermodynamic phase
(MOD06, L3 0.1° gridded, Terra, 21 Nov 2003;
modis-atmos.gsfc.nasa.gov)
Uncertain
Ice
Liquid
S. Platnick, SORCE, 5 Dec 2003
MODIS Cloud Product – optical thickness
(MOD06, L3 0.1° gridded, Terra, 21 Nov 2003;
modis-atmos.gsfc.nasa.gov)
S. Platnick, SORCE, 5 Dec 2003
MODIS Cloud Product – particle effective radius
(MOD06, L3 0.1° gridded, Terra, 21 Nov 2003;
modis-atmos.gsfc.nasa.gov)
S. Platnick, SORCE, 5 Dec 2003
MODTRAN, absorption transmittance only
MISR (Terra)
nominal bands locations
9 cameras
± 70 deg views
S. Platnick, SORCE, 5 Dec 2003
MISR 9-camera animation over southern Florida
(true-color composite)
Click to See Movie
S. Platnick, SORCE, 5 Dec 2003
MODTRAN, absorption transmittance only
polarization
channels
POLDER
(CNES, ADEOS-I,II)
CCD array, rotating filter
wheel
S. Platnick, SORCE, 5 Dec 2003
Cloud Observations with AirPOLDER
(19 minutes of data, Proteus Aircraft, CRYSTAL-FACE, 3 July 2002)
1520 UTC
1539
S. Platnick, SORCE, 5 Dec 2003
Cloud Observations with AirPOLDER
(19 minutes of data, Proteus Aircraft, CRYSTAL-FACE, 3 July 2002)
QuickTime™ and a
Video decompressor
are needed to see this picture.
QuickTime™ and a
Video decompressor
are needed to see this picture.
Click to See Movie
Click to See Movie
Total radiance RGB: 865, 763, 443 nm
RGB: 865(pol), 865(total), 763(total)
(figs. courtesy of Jerome Riedi, U. Lille, France)
S. Platnick, SORCE, 5 Dec 2003
Calibration for reflectance-based remote sensing
Fundamental measurement is bidirectional reflectance not radiance,
defined for some spectral band λ as:
Rλ (θ,θ 0 ) =
where,
π Iλ
cos(θ 0 )F0,λ
θ = viewing zenith angle, θ0 = solar zenith angle
Iλ(θ) = spectral radiance (intensity) measured in viewing direction
F0,λ = solar spectral irradiance
Calibration approaches:
1. Radiance calibration + solar spectral irradiance table —> reflectance
* 2. On-board reflectance calibration (e.g., MODIS, MERIS, etc.)
* 3. Other: vicarious calibration (ground-based validation), lunar
observations, inter-satellite comparisons, etc.
* useful for stability as well as absolute cal
S. Platnick, SORCE, 5 Dec 2003
1. Radiance-based approach
Reflectance uncertainty is:
dIλ dF0 λ
dRλ
=
−
Rλ
Iλ
F0 λ
Iλ difficulties compared with F0,λ :
– Lack of spaceborne absolute radiometery for imagers (e.g,
absolute detectors, electrical substitution radiometers)
low energy(narrowband), short pixel dwell time (especially
scanners, ~300 µs for MODIS 1km bands)
even if possible (microbolometer), would have to measure solid
angle FOV in addition to aperture area
– Difficulty in transferring standards, e.g., standard irradiance lamp
transferred to radiance via diffuse plate to integrating sphere
– Fortunately, remote sensing needs typically much less stringent
than energy budget measurements (though stability still critical!)
S. Platnick, SORCE, 5 Dec 2003
Integrating Sphere calibration intercomparison
(relative to SBRS SIS100 sphere cal)
15
4
EOS
EOSVXR
VXR
VNIR
UAUAVNIR
GSFC LXR
LXR
GSFC
10
(f)
100 (LXR/LSIS - 1)
100 (LXR/LSIS - 1)
-66
2
0
-2
-4
5
0
-5
-10
-15
-20
-25
-30
-6
400
500
600
700
wavelength (nm)
800
900
(e)
-35
00 800
EOSSWIXR
SWIXR
EOS
UA
SWIR
UA SWIR
1000 1200
1400 1600
1800 2000
2200 2400
wavelength (nm)
water vapor bands
Butler et al., J. Res. NIST, 108, May-June 2003.
(figs. courtesy of Jim Butler, NASA GSFC)
S. Platnick, SORCE, 5 Dec 2003
1. Radiance-based approach, cont.
Reflectance uncertainty is:
dIλ dF0 λ
dRλ
=
−
Rλ
Iλ
F0 λ
F0,λ from published compilations and/or measurements:
MODTRAN
MODIS
ASTERuse
personal
Landsat
ETM+
(backup
to refl.
cal.)
• 1974 NASA spectrum (Thekaekara, 1974 ): UV/VIS [CV-990 flights,
Thekaekara (1969), JPL a/c program, Drummond (1967-68)], NIR-MWIR [3
published papers]
• 1981 WRC spectrum: 0.3-1.25 µm [Neckel and Labs (1981) Jungfraujoch,
spectral improvement, absolute pinned to WRC solar constant], Other
spectral regions [Smith and Gottlieb (1974), Heath and Thekaekara
(1977), Arvesen et al. (1969)]
• 1984, Neckel and Labs: 0.33-1.25 µm (improved spectral w/Kitt Peak
FTS, not absolute)
• 1995, Kurucz: UV-SWIR compilation using Jungfraujoch, Kitt Peak,
JPL/ATMOS, …; adopted by MODTRAN
• 1998, 2002, Thuillier et al.: UV-SWIR, ATLAS SOLSPEC, SOSP EURECA
• 20??: SORCE SIM
S. Platnick, SORCE, 5 Dec 2003
MODIS band-averaged reflectance difference
relative to MODTRAN solar irradiance spectrum (Kurucz)
S. Platnick, SORCE, 5 Dec 2003
1. Radiance-based approach, cont.
NOTE: A very uncomfortable uncertainty in the 3.7 µm band solar
irradiance! Data sources include (?):
• Thekaekara et al. (1974) – at 100 nm spectral resolution
• Kondratyev, Andreev, Badinov, Grishechkin, and Popova (1965) – at
3.0, 3.6, 4.0 µm
• ? Farmer and Norton (1989), Farmer et al. (1994), Livingston and
Wallace (1991)
Example comparison between KABGP & Thekaekara et al. at 3.6
µm shows irradiance difference of about 15%, e.g.,
Thekaekara et al. = 1.4 mW-cm-2-µm-1
KABGP
= 1.2 mW-cm-2-µm-1
S. Platnick, SORCE, 5 Dec 2003
MODIS Terra granule
coastal Chile/Peru (18 July 2001, 1530 UTC)
RGB true-color composite
phase
retrieval
uncertain ice
liquid
no
water retrieval
S. Platnick, SORCE, 5 Dec 2003
MODIS Terra granule, coastal Chile/Peru (18 July 2001, 1530 UTC)
40
-1.0
32
24
-1.5
16
8
0
-2.0
3.7 µm retrieved re
ice clouds
(Thekaekara)
∆re (KABGP - Thekaekara)
S. Platnick, SORCE, 5 Dec 2003
2. Reflectance-based approach
(MODIS example, VIS-SWIR)
calibration schematic
Sun
1.4 % screen
MODIS Spectralon diffuser panel
optional 7.8 %
screen (bands
8-16 saturate
w/o screen)
58.1°
SDSM
to
scan mirror
20.5°
20.7°
SD
MODIS Solar Diffuser Stability Monitor
instrument (integrating sphere, 9 filters,
0.4-1 µm; views sun w/screen & panel)
S. Platnick, SORCE, 5 Dec 2003
Difference relative to NIST (%)
Laboratory panel BRDF measurements (relative to NIST)
Spectralon at λ=633 nm
4
4
3
3
2
2
1
1
0
0
-1
-1
-2
-2
(c) θi = 45°
-3
-4
-60
-40
-20
0
20
-3
(d) θi = 60°
-4
40
60
-60 -40 -20
viewing angle
Viewing
Angle(deg)
[deg]
Laboratory
Laboratory
GSFC
SBRS
JPL
UA
0
20
40
60
Early et al., J. Atmos. Oceanic Tech., 17, August 2000.
(figs. courtesy of Jim Butler, NASA GSFC)
S. Platnick, SORCE, 5 Dec 2003
MODIS Solar Diffuser Degradation
(fig. courtesy of Bill Barnes, Jack Xiong, NASA GSFC)
S. Platnick, SORCE, 5 Dec 2003
Satellite Instruments w/Solar Diffusers (incomplete?)
• Used for primary calibration
– MODIS, MERIS (?)
• Used for trending
– MISR, SeaWiFS (primary methods are vicarious calibration)
• Not used
– ETM+ (due to apparent diffuser degradation relative to vicarious
calibration and pre-flight cal)
S. Platnick, SORCE, 5 Dec 2003
Solar Remote Sensing Summary
• Fundamental measurement needed for geophysical retrievals typically
reflectance (not radiance)
• Absolute calibration not as stringent as irradiance energy budget
requirements, but stability critical for climate monitoring
• New generation of satellite sensors w/on-board solar reflectance
panels, flown with varying degrees of success
• Accurate solar spectral irradiance needed across the solar spectrum
– Radiance-based calibration methods —> reflectance
– Intercomparison of reflectance and radiance-based methods
– Traceability of reflectance-based radiometry to MKS standards
• 3.7 µm band for cloud re retrievals: heritage(AVHRR) and new
(MODIS, CERES group) studies subject to unknown solar irradiance
uncertainty
S. Platnick, SORCE, 5 Dec 2003